Disease relapse is the most common reason for treatment failure of both autologous (auto) and allogeneic (alio) blood and marrow transplantation (BMT). As such, there is the unmet need to augment antitumor immunity in these settings. We propose a novel adoptive cell therapy (ACT) approach to augment antitumor immunity after autoBMT and to treat the post-transplant relapse after alloBMT by exploiting the unique characteristic of the bone marrow as both the primary anatomic site for most hematologic malignancies and a compartment enriched with tumor-reactive marrow infiltrating lymphocytes (MILs). We hypothesize that ex vivo activated tumor-specific MILs can impart measurable and sustainable antitumor immunity upon adoptive transfer. This hypothesis is formulated on the basis of our preliminary data and by bringing together innovative strategies developed during the previous funding cycle. MILs from multiple myeloma patients can be expanded ex vivo with anti-CD3/CD28 stimulation and activated as to significantly Increase their tumor specificity in ACT studies. Similarly, MILs obtained from patients undergoing alloBMT using PTCy-based GVHD prophylaxis can also be expanded, using the same techniques and augment their antitumor reactivity. Accordingly, in Specific Aim #1, we will determine if activated MILs in combination with an allogeneic myeloma cell vaccine or lenalidomide can augment and/or sustain antitumor immunity after autoBMT and assess the impact of activated MILs on immune reconstitution, tumor-specific immunity and correlate these parameters with clinical responses. In Specific Aim #2, we will conduct a phase l/ll clinical trial to evaluate the feasibility/safety of alloMILs obtained from the patient as a more tumor-specific DLI in patients with myeloid and lymphoid malignancies relapsing after alloBMT following PTCy and examine the impact of activated alloMILs on immune reconstitution, risk of GVHD, and tumor-specificity. Finally, in Specific Aim #3 we will characterize the effects of lenalidomide on modulating MILs' intrinsic differentiation program, expansion, survival, and extrinsic Inhibitory signals and determine the effects of coinhibitory molecules on the ability of MILs to augment antitumor immunity. In aggregate, the proposed research is significant in addressing an important unmet need of reducing or treating relapsed disease following BMT through clinical studies as well as increasing the overall understanding of mechanisms of antitumor immunity and developing innovative ACT strategies utilizing MILs to augment antitumor immunity posttransplant.